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Those inverters you don't know

Burning two new motors together! The output current and voltage of the inverter are balanced, the input voltage is balanced, the current is unstable, and the fluctuation is between 20% and 50%. The inverter is burned out at 35Hz short-circuit, and the grid voltage is very stable, 720V, The distance between the motor and the inverter is 120m, and reactors are added to the input and output ends of the inverter. The motor current is very small, and the ambient temperature is about -10 degrees, which can eliminate the cause of the motor overheating. What causes this situation to cause a motor failure?
Analysis reason: The current of the load motor is not large. It is caused by the inverter. The inverter is too far away from the motor. The harmonics of the output current of the inverter break down the insulation between the motors and cause a short circuit. The quality of the voltage.
There is a certain distributed capacitance between the cable and the ground. This capacitance will affect the propagation of electricity, cause a certain phase shift between the current and the voltage, and make the motor (peak, oscilloscope can Seen that ordinary multimeters cannot see) high voltage, high voltage breakdown of motor windings.
Solution: if the voltage is not enough to burn the motor, you can use an international certification (CE, UL) inverter + suitable reactor can avoid such problems.
1. Does the inverter affect the motor?
Ordinary asynchronous motors are designed according to constant frequency and constant voltage. It is impossible to fully meet the requirements of variable frequency speed regulation. The following is the impact of the inverter on the motor:
1. The efficiency and temperature rise of the motor
No matter what kind of frequency converter, it generates different levels of harmonic voltage and current during operation, which makes the motor run under non-sinusoidal voltage and current. According to the data, taking the commonly used sine wave PWM inverter as an example, its low-order harmonics are basically zero, and the remaining high-order harmonic components that are about twice as large as the carrier frequency are: 2u + 1 (u Is the modulation ratio).
Higher harmonics will increase the copper loss of the stator of the motor, copper (aluminum) loss of the rotor, iron loss and additional loss. The most significant is the copper (aluminum) loss of the rotor. Because the asynchronous motor rotates at a synchronous speed that is close to the frequency of the fundamental wave, a high rotor voltage will cause a large rotor loss after cutting the rotor guide bar with a large slip. In addition, additional copper losses due to skin effect need to be considered. These losses will cause the motor to generate extra heat, reduce efficiency, and reduce output power. For example, if a normal three-phase asynchronous motor is operated under the non-sinusoidal power supply output of the inverter, its temperature rise will generally increase by 10% -20%.
2. Motor insulation strength
At present, many small and medium frequency converters use PWM control. Its carrier frequency is about several thousand to several ten kilohertz rise rate, which is equivalent to applying a steep shock voltage to the motor, which makes the interturn insulation of the motor withstand harsher 4-6 times the voltage superimposed on the motor operating voltage. It will pose a threat to the motor's insulation to the ground. In addition to the high voltage, the stator windings of the motor must withstand high voltages. The rectangular chopping impulse generated by the PWM frequency converter accelerates aging.
3. Harmonic electromagnetic noise and vibration
When ordinary asynchronous motors are powered by inverters, vibration and noise caused by electromagnetic, mechanical, and ventilation factors will become more complicated. Each time harmonic contained in the variable frequency power supply interferes with the inherent space harmonics of the electromagnetic part of the motor to form various electromagnetic excitation forces. When the frequency of the electromagnetic force wave is the same as or close to the natural vibration frequency of the motor body, a resonance phenomenon will occur, thereby increasing the noise. Because the motor has a wide operating frequency range and a large speed variation range, it is difficult for the frequency of various electromagnetic force waves to avoid the natural vibration frequency of each component of the motor.
4.The adaptability of the motor to frequent starting and braking
After the inverter is used to supply power, the motor can be started at a very low frequency and voltage without shock current, and it can use various braking methods provided by the inverter to perform rapid braking. Conditions have been created, so the mechanical system and electromagnetic system of the motor are under the effect of cyclic alternating force, which brings fatigue and accelerated aging problems to the mechanical structure and the insulation structure.
Second, the inverter will generate high odd harmonics.
Mainly the 5th and 7th times have a greater impact on the inverter and the motor. Usually when designing to reduce the impact of harmonics, reactors, absorption capacitors, etc. will be added. A filter can also be added at the output of the frequency converter.
How to calculate the harmonic power of the inverter-powered motor?
method one:
Fourier transform obtains the amplitude and phase of each harmonic of voltage and current, calculates the active power of each harmonic according to P = √3UIcosφ, and adds the active power of all harmonics to obtain the harmonic power.
Method Two:
The total active power is measured. The fundamental wave amplitude and phase of the voltage and current are obtained by Fourier transform. The fundamental active power is calculated according to P = √3UIcosφ. The total active power minus the fundamental active power is the harmonic power.
Harmonic power measurement accuracy is low. Generally, the higher the harmonic frequency, the lower the accuracy. The second method is recommended.
 
Third, the causes of inverter harmonics on the motor and improvement methods.
1. The reason for the motor damage is that the inverter generates a high peak voltage on the stator windings of the motor. The amplitude of the peak voltage exceeds the insulation strength of the windings, resulting in winding damage. The amplitude of the spike voltage can reach more than three times the rated operating voltage of the inverter. For example, for an inverter with a rated voltage of 380V, the amplitude of the spike voltage exceeds 1200V. This peak voltage impacts the stator windings of the motor thousands of times per second, which will soon cause damage to the stator windings.
2. The cause of motor damage is that the inverter will also generate bearing current in the bearing of the motor. If the bearing current flows in the bearing for a long time, it will cause the bearing to burn out. The smaller the power of the motor, the more easily the stator winding will be damaged. Motors, the more easily the bearings are damaged.
Weak protection: Since all commercially available motor protectors now collect the value of current or voltage changes, the purpose of protecting the motor is achieved; however, motor bearings are damaged due to various reasons, and the rotor is eccentric, which causes the motor to clean the chamber. The problem of burning the motor. These protectors can no longer function as protection, because only when the winding is burned out and the motor is short-circuited, this kind of protector will work, but it is too late; so far there is no An intelligent protection product for motor bearings; many users can only artificially protect some large motors by manually monitoring or periodically inspecting the temperature changes at the test bearings. There are two disadvantages to this approach:
1) Increased personnel workload, increased personnel costs of the enterprise, and unable to care for all motors.
2) There is a time limit for manual testing. It is impossible to stay away from people within 24 hours. If the bearing is damaged during non-testing time, the rotor will be eccentric, the motor will be bored, and the accident of burning the motor will not be avoided When driven by an inverter, the life is greatly reduced, and in severe cases, stator winding damage may occur within a few months. The resulting suspension caused huge losses to the enterprise.